Crystal Structure of Cyclophilin A Complexed with a Fragment of HIV-1 Gag Protein. Yingdong Zhao⁺, Yongquan Chen⁺, Mike Schutkowski^[daggerdbl], Gunter Fischer^[daggerdbl], and Hengming Ke⁺, ⁺Department of Biochemistry and Biophysics, School of Medicine, The University of North Carolina, Chapel Hill, NC 27599, USA, ^[daggerdbl]Max-Planck-Gesellschatt zur Förderung der Wissenschaften, E.V., Arbeitsgruppe "Enzymologie der Peptidbindung", Kurt-Mothes-StraBe 3, D-06120 Halle/Saale, Germany

Cyclophilin A (CyPA), a receptor of the immunosuppressive drug cyclosporin A (CsA) and also an enzyme catalyzing cis-trans isomerization of peptidyl-prolyl bonds, is required for infectious activity of human immunodeficiency virus type 1 (HIV-1).

The crystal structure of CyPA complexed with a 25 amino acid peptide of HIV-1 gag capsid protein (25mer) was determined and refined to an R-factor of 0.195 at 1.8 Å resolution. The sequence Ala88-Gly89-Pro90-Ile91 of the gag fragment is the major portion to bind to the active site of CyPA. The binding of Pro90-Ile91 is similar to Pro-Phe of the substrate succinyl-Ala-Ala-Pro-Phe-p-nitroanilide (AAPF), while the N-terminal Ala88-Gly89 has a different hydrogen bonding pattern and molecular conformation from AAPF. The peptidyl-prolyl bond between Gly89 and Pro90 has a trans conformation, in contrast to the cis conformation of other known CyPA-peptide complexes. The residue preceding proline, Gly89, has an unfavorable backbone conformation usually only adopted by glycine.

The unfavored backbone conformation of Gly89 of the gag 25mer fragment suggests that binding between HIV-1 gag protein and CyPA requires a special sequence Gly-Pro. Thus, CyPA is likely to function as a chaperone, rather than as a cis-trans isomerase, in HIV-1 infectivity. However, the involvement of cis-trans isomerization of CyP cannot be completely ruled out, because of similarities between the C-termini of the 25mer and the substrate AAPF.